1. Field of the Invention
The invention relates to apparatus for molding thermoplastic tubing in which a sizing plug is used to define internal diameter of the tubing. The invention is especially useful in the production of ribbed or double walled tubing.
2. Description of prior art
The apparatus may be of the type in which tubing of thermoplastic material is continuously extruded into a travelling tubular mold tunnel about a mandrel and is confined to the shape of the pipe by a sizing plug downstream of the mandrel. Such a plug may, on some occasions, when extrudate does not easily flow fully into the mold, be heated to increase the fluidity of the extrudate so that it may more easily flow into recesses of the mold, for example under the influence of suction from the bases of the recesses. However, more usually the extrudate is provided in sufficiently molten condition to flow fully into the mold. In this case it is necessary to provide a sizing plug to define the inner wall of the pipe so as to confine the extrudate in an appropriate casting cavity.
A simple sizing plug defining the inner wall of a tube is described by Chaplain in U.S. Pat. No. 4,365,948 issued Dec. 28, 1982. Usually some temperature control is desirable in a sizing plug and Lupke in U.S. Pat. No. 4,545,751 issued Oct. 8, 1985 describes a more sophisticated arrangement. In that arrangement the sizing plug does not define an inner wall of the pipe but is used to distribute air into the inside of corrugated tubing. Pressurized air is delivered through a central core coaxial with an extrusion nozzle. Hot peripheral air is drawn off and central cool air is permitted to leave the plug to contact the inside of corrugated pipe, the plug being inwardly spaced from the inner pipe wall. Such arrangement is suitable for use where corrugated tubing is involved but the range of temperature in cooling may be limited. The arrangement is less suitable where ribbed tubing is to be produced since the plug, in that case, should be a plug contacting the inside wall of the pipe to confine it into a casting region. When this is the case, air cannot be released between the plug wall and the inner pipe wall for cooling. A cooling plug suitable for use in the formulation of ribbed pipe is disclosed by Lupke in his later U.S. Pat. No. 4,555,230 issued Nov. 26, 1985. In that patent Lupke does not attempt to use released cooling air but provides within and near the surface of a follower plug, a helical coil of tubing through which cooling fluid may be passed.
Apparatus for the production of plain single walled tube with no ribs or corrugations usually comprises means for extruding a parison into a mold the temperature of which may be adjusted by the use of a water jacket. No sizing plug is used to form the inner surface of the tube but gas pressure or the like may be used to press the tube against the outer mold. The inner wall of tube thus formed may not be totally even due to a variety of reasons such as uneven shrinkage during setting of the tube. Moreover, it would be advantageous to be able to produce such tube at greater speeds.
A sizing plug is provided to form a smooth inner wall to the formed tube but, when ribbed pipe is formed, some problems have occurred in obtaining a smooth inner wall. The reason is due to the variation in the requirement of plastic material due to the varying wall thickness for the ribs and troughs. Thus, more plastic material will be required to form the ribs than that part of the tube wall (trough) between ribs. Stresses are set up in the apparatus at the point of filling the mold blocks due to this varying demand for plastic material. Moreover, unevenness due to shrinkage is accentuated where thickness of the tube varies, as for example, where there are ribs and troughs.
U.S. Pat. No. 5,023,029 of the same inventor and issued Jun. 11, 1991, to Manfred A. A. Lupke, describes and claims the use of an accumulator chamber for plastic material at the point of mold filling. The accumulator chamber is intended to even out pressure variations and reduce stresses. The use of such an accumulator chamber is very useful in its intended purpose but there still may be some effects of pressure variations to be seen on the inside walls of the tube.
Thus, the inside wall of the tube may have a slight wave formation, tending to apparently bulge inwardly between the ribs where the pressure of extrudate is greater, and tending to actually recede to form an indentation opposite a rib where the pressure of extrudate is less. This effect is accentuated when an accumulator chamber is not used.
When double walled corrugated tubing is to be produced difficulties have been encountered due to pressure within the corrugations. If this pressure is too low, the inner wall may bow into the corrugation forming an undesirable concavity in the inner wall. If the pressure is too high, the inner wall may bow away from the corrugation forming an undesirable bulge in the inner wall.
U.S. Pat. No. 5,186,878 describes and claims method of applying vacuum applied at the surface of a cooling plug to mitigate rhythmic bulges and indentations in the inner tube wall.
U.S. Pat. No. 4,808,098 issued Feb. 28th, 1989 to Chan et al discloses a cooling plug having two annular distributing grooves, the grooves being independently connected to a vacuum source and one groove being axially remote and the other being axially close to the extrusion orifice, whereby the intensity of vacuum along the plug may be varied.
The present invention relates to an improvement in the method of U.S. patent application Ser. No. 860,269 whereby suction is alternated with pressure, when, for example, bells are required in the tube.